This invention relates generally to pulp and paper making, and more particularly to the bleaching and delignification of pulp.
There are three major types of pulping methods known in the Pulp and Paper Industry. The first is Chemical and the second is Mechanical and the third is a Combination of Chemical and Mechanical. Methods to enhance the effectiveness of all three types of pulping methods are always desirable.
In Chemical pulps, sufficient lignin is dissolved to allow the fibers to separate with little, if any mechanical action. However, a portion of the lignin remains with the fiber and an attempt to remove this during digestion would result in excess degradation of the pulp. The degradation is a depolymerization of the cellulose and is measured by determining the viscosity of the cellulose dissolved in special solvents. For this reason from about 3 wt. % lignin to about 4 wt. % lignin is normally left in hardwood chemical pulps and from about 4 wt. % to about 10 wt. % lignin is normally left in softwood chemical pulps after the cook or digestion. The lignin is subsequently removed by bleaching in separate pulp mill operations if completely delignified and whitened pulps are to be produced.
The dominant Chemical wood pulping process is the kraft (xe2x80x9ckraftxe2x80x9d means strength in German) or sulfate process. In the kraft process, the alkaline pulping liquor or digesting solution contains about a 3 to 1 ratio of sodium hydroxide and sodium sulfide. A stronger pulp is obtained when sodium sulfide is used in combination with sodium hydroxide. This is to be compared with pulp obtained when sodium hydroxide is used alone, as it was in the original soda process. In the kraft process the wood is delignified (pulped) with a solution of sodium hydroxide and the addition of sodium sulfide is beneficial for pulping as well. Key advantages of the kraft process is its great adaptability of pulping many different species of wood and yielding pulps that may be used for a variety of applications.
Another type of Chemical pulping is the xe2x80x9csulfite processxe2x80x9d. The sulfite process has several advantages over the kraft process. These advantages include improved yield (45-55%), lower cost cooking chemicals, higher brightness pulps and more easily bleached pulps. However, the sulfite method also has two distinct disadvantages: only a limited number of species can be pulped and the pulps produced are distinctly weaker than those made using the kraft or sulfate process.
In Mechanical pulping, pulp is made predominantly using mechanical methods. The fundamental criteria used in assessing the quality of mechanical pulp is the amount of energy expended per unit of production. Because this energy is difficult to quantify, pulp freeness is most commonly used as a process control parameter. Generally, the more the energy expenditure the lower the freeness of the pulp.
The first step in the Mechanical pulping process is the grinding or refining of wood.
The Stone Groundwood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Many years ago the grinding surface used was an actual stone. In current practice specifically designed xe2x80x9cartificial pulp stonesxe2x80x9d are available for the grinding.
A Pressurized GroundWood (PGW) process is where the grinding operation is completely pressurized.
Another type of Mechanical pulping is Refiner Mechanical Pulp (RMP) featuring atmospheric refining with no pretreatment of the wood chips. This process is one of the main mechanical pulping operations.
Thermo Mechanical Pulping (TMP) is a Mechanical pulping process that evolved from RMP and a high temperature process known as the Apslund process. Thermo Refiner Mechanical Pulping (TRMP) is a variation in Thermo Mechanical Pulping. In this case, the chips are preheated under pressure and refining is carried out at atmospheric pressure. TMP and TRMP pulps are stronger than either SCW or RMP pulps.
The third type of pulping process is a Combination of Chemical and Mechanical pulping processes. Two types of Combination processes are ChemiMechanical Pulping and SemiMechanical Pulping. There is little difference between ChemiMechanical Pulping (CMP) and SemiChemical Mechanical Pulping (SCMP). Both processes involve pretreatment of chips with chemicals, followed by mechanical refining. Four different chemical treatments are associated with these processes. These chemical treatments are: sodium hydroxide, sodium bisulfite, sodium sulfite, acid sulfite treatment. These processes are generally used on hardwoods. Chemical treatment weakens the fiber structure allowing fibers to rupture similarly to softwood that is mechanically pulped.
ChemiThermoMechanical Pulping (CTMP) appears to be a full evolution of all Mechanical pulping methods. It includes chemical treatment elevated temperature steaming followed by mechanical refining. This process can produce fibrous raw materials that vary considerably in properties depending upon process conditions such as sodium sulfite concentration, pH, temperature, etc.
With all pulps, xe2x80x9cpulp brightnessxe2x80x9d is a measurement of the ability of a sample to reflect monochromatic (457 nm) light as compared to a known standard, using magnesium oxide (MgO). Since cellulose and hemicellulose are white, they do not contribute to pulp color. It is generally agreed that the lignin left in the pulp after pulping is responsible for the color the pulp. This unbleached pulp has an appearance similar to brown grocery bags. The chromophores are believed to be quinone-like materials formed from the lignin""s phenolic groups through an oxidative mechanism. Additionally, heavy metal ions, especially iron and copper, can form colored complexes with the phenolic groups.
There are generally two approaches to removing color. The first uses a selective chemical to destroy the chromophores but not the lignin. The other approach is to use a bleaching system to remove the residual lignin. The bleaching of pulp is the standard method of removing color from pulp. It is current state of the art technology for all Chemical and Mechanical pulps to be bleached.
The bleaching of pulp and the subsequent delignification of pulp is usually performed in several stages, with each stage being referred to by a letter designation. Note, that although all pulps are bleached, only Chemical pulps are delignified using oxygen treatment.
The following table briefly describes the most common stages in a xe2x80x9ctypicalxe2x80x9d bleaching process. Note that the stages captured in this table are not necessarily in the order that they are practiced. For example, oxygen delignification is typically never the last step in the process as oxygen delignification leaves the pulp yellowish in color. That is why oxygen delignification is followed by some level of bleaching.
Usually the chlorination and extraction stages are carried out in sequence, first chlorinating the lignin compounds and then solubilizing them in the alkaline extraction stage. This is similar to the oxygen stage in that the objective is exclusively to delignify the pulp.
Five or six stages are needed to produce a xe2x80x9cfull bleachxe2x80x9d brightness level of 89 to 91% MgO. Most commonly these stages, in order are CEDED, CEHDED and OCEDED. A brightness of 65% MgO can be obtained with less stages, usually a CEH. Intermediate brightness levels can be reached using CED, CEHH, CEHD, or CEHP. Brightness enhancement during bleaching of pulp, as well as improving selective lignin removal during oxygen delignification of the chemical (kraft) pulp is important in the pulp and paper industry. Brightness enhancement is also useful in Mechanical pulps.
It is to be understood that separate from the technical aspects of bleaching pulp there are environmental concerns that have dictated that chlorination is being rapidly reduced or eliminated in favor of alternative treatments.
In current practice in pulp and paper mills, Mechanical pulps are not oxygen delignified.
Currently, hydrogen peroxide is the dominant bleaching agent for mechanical pulps. Sodium hydrosulfite can also be used for bleaching. Several auxiliary chemicals are needed to provide an adequate performance. These auxiliary chemicals include sodium silicate for stability and chelation, sodium hydroxide for alkalinity, chelating agents such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA) for control of heavy metals, and magnesium sulfate for cellulose stability. Each chemical added increases the cost of the bleaching method.
Although the benefits of using a chelant are known in the pulp and paper industry, the known chelants used in hydrogen peroxide bleaching:
(1) are usually selective in regards of the target transition metal ions (e.g., removing manganese but not iron or vice versa);
(2) must be applied in substantial quantities to achieve a noticeable effect; and
(3) require washing out complexes formed during the treatment.
Selectivity can be defined as the ratio of the change in delignification (kappa number that characterizes lignin removal; the lower the better) divided by the change in viscosity (that characterizes carbohydrate depolymerization, the higher the better). Currently, the commonly used chemical for increasing the selectivity of oxygen delignification is magnesium sulfate. Magnesium sulfate does not influence the delignification, but provides a small measure of protection for the pulp viscosity. Conventional chelants such as DTPA and EDTA are also used for this purpose, however, none of them are reported to affect the kappa number. Therefore, currently, there are no known additives that can provide a noticeable improvement in lignin removal during oxygen delignification.
Japanese Patent Application No. 4-114853 discloses a method of pretreatment of wood pulp before bleaching which uses certain water-soluble polymers. The desired goal is to subject wood pulp to bleaching pretreatment through inexpensive pretreatment having little toxicity, thereby permitting a high degree of bleaching of wood pulp in the subsequent bleaching step. The recommended amount of polymer used in this Japanese Patent Application is from about 0.04 to 0.8 wt % per xe2x80x9cexsiccatedxe2x80x9d pulp.
It would be desirable to identify additional or alternative compounds capable of enhancing the brightening of pulp in Chemical and Mechanical Pulping processes; or to be capable of enhancing delignification of pulp in Chemical Pulping processes.
The first aspect of the instant claimed invention is, in a method for making Chemical pulp comprising the steps of digesting wood chips in the digester to create unbleached pulp and then bleaching the pulp and optionally subjecting the pulp to pressurized oxygen delignification; the improvement comprising treating the pulp with from about 0.002 weight % to about 0.02 weight % of an organic sulfide chelating agent before or during bleaching, or before optional oxygen delignification of the pulp.
The second aspect of the instant claimed invention is in a method for making Mechanical pulp comprising the steps of grinding or refining wood to create unbleached pulp and then bleaching the pulp; the improvement comprising treating the pulp with from about 0.002 weight % to about 0.02 weight % of an organic sulfide chelating agent before or during bleaching.
Throughout this patent application, the following terms have the indicated meanings.
Aldrich refers to Aldrich, P.O. Box 2060, Milwaukee, Wis. 53201.
AMP refers to aminotris(methylenephosphonic acid).
CAS Registry Number is the Chemical Abstracts Services Registry Number.
CMP refers to ChemiMechanical Pulping.
Consistency=wt. in grams of oven-dry fiber
xe2x80x83100 grams of pulp-water mixture
This definition of consistency is according to Tappi recommended procedure T240om-93.
Throughout this patent application, consistency is stated either as a decimal number or as the equivalent percentage.
CTMP refers to ChemiThermoMechanical Pulping.
DTPA refers to diethylenetriaminepentaacetic acid.
EDTA refers to ethylenediaminetetraacetic acid.
MgO refers to magnesium oxide.
Nalco refers to ONDEO Nalco Company, ONDEO Nalco Center, 1601 W. Diehl Road, Naperville, Ill. 60563, (630) 305-1000. 
PAPEMP refers to polyaminopolyethermethylenephosphonate.
PGW refers to Pressurized GroundWood pulp.
RMP refers to Refiner Mechanical Pulp.
SCMP refers to SemiChemical Mechanical Pulping.
SGW refers to Stone GroundWood pulp.
TDA refers to tridecylalcohol.
TMP refers to Thermo Mechanical Pulping.
TRMP refers to Thermo Refiner Mechanical Pulping.
Weight % means weight of active ingredient per weight of dry pulp.
The first aspect of the instant claimed invention is in a method for making Chemical pulp comprising the steps of digesting wood chips in the digester to create unbleached pulp and then bleaching the pulp and optionally subjecting the pulp to pressurized oxygen delignification; the improvement comprising treating the pulp with an organic sulfide chelating agent either before or during bleaching or before oxygen delignification of the pulp.
The Chemical pulp can be one of several types including kraft, sulfite, chemimechanical and ChemiThermoMechanical Pulp.
The method is comprised of treating the Chemical or Mechanical pulp with an organic sulfide chelant selected from the group consisting of monomeric dithiocarbamates, polymeric dithiocarbamates, polydiallylamine dithiocarbamates, 2,4,6-trimercapto-1,3,5-triazine, thiocarboxylic acid, thioglycolic acid, sodium trithiocarbonate, mercaptoquinazolinone, mercatopyridine, mercatopyrimidine, thiolactic acid, mercaptoethanol, mercaptopropanol, 2,3-dimercaptopropanol, thioglycerol, oxydiethanethiol, disodium ethylenebisdithiocarbamate, dithiothreitol, benzenethiol, mercaptoimidazole, mercaptobenzimidazole, mercaptotriazole, mercaptotetrazole and salts thereof and mixtures thereof.
Thioglycolic acid is available from Aldrich.
Thiolactic acid is available from Aldrich.
Mercaptoethanol and mercaptopropanol are available from Aldrich
2,3-dimercaptopropanol is available from Aldrich.
Thioglycerol is available from Aldrich.
Dithiothreitol is available from Aldrich.
Thiophenols are available from Aldrich.
Mercaptoimidazole is available from Aldrich.
Mercaptobenzimidazole is available from Aldrich.
Mercaptotriazole is available from Aldrich.
Mercaptotetrazole is available from Aldrich.
Mercaptoquinazolininone is available from Aldrich.
Mercaptopyridine is available from Aldrich.
Mercaptopyrimidine is available from Aldrich.
Mercaptothiazoline is available from Aldrich.
Disodium ethylenebisdithiocarbamate is available from Alco Chemical of Chattanooga of Tennessee. 2,4,6-Trimerapto-1,3,5-triazine trisodium salt is avaiable from Degussa-Huls AG of Germany.
Polydiallylamine dithiocarbamates useful in the instant claimed invention are water-soluble polymers of Formula A: 
wherein R is H or CSxe2x88x922 X+ and X+ is an alkali metal (such as sodium or potassium), an alkaline earth metal or ammonium, and n is the number of repeating units such that the polymer has a total molecular weight in the range of from about 3000 to about 100,000. Polymers of Formula A are described and claimed in U.S. patent application Ser. No. 09/638,434, filed Aug. 14, 2000, entitled, xe2x80x9cWater Soluble Polymer Containing Dithiocarbamate Functionalitiesxe2x80x9d by William S. Ward. U.S. patent application Ser. No. 09/638,434 is incorporated by reference in its entirety.
Monomeric dithiocarbamates and polymeric dithiocarbamates are known to people of ordinary skill in the art. They can be synthesized and some of them are available commercially from Nalco.
The preferred organic sulfide chelants for use in the method of the instant claimed invention are monomeric dithiocarbamates and polymeric dithiocarbamates.
Preferable monomeric dithiocarbamates are of Formula I and Formula II. 
In Formula I, R1 and R2 can be the same or different and are selected from the group consisting of methyl, ethyl and propyl. R3 is a positive metal cation selected from the group consisting of sodium and potassium.
In Formula II, R4 is selected from the group consisting of methyl, ethyl and propyl. R3 is positive metal cation selected from the group consisting of sodium and potassium.
Most preferred monomeric dithiocarbamates of Formula II and Formula I are:
Carbamodithioic acid, methyl-, monosodium salt, CAS Reg. No. 137-42-8;
Carbamodithioic acid, methyl-, monopotassium salt, CAS Reg. No. 137-41-7;
Carbamodithioic acid, ethyl-, monosodium salt, CAS Reg. No. 13036-87-8;
Carbamodithioic acid, ethyl-, monopotassium salt, CAS Reg. No. 63467-57-2;
Carbamodithioic acid, propyl-, monosodium salt, CAS Reg. No. 20562-43-0;
Carbamodithioic acid, propyl-, monopotassium salt, CAS Reg. No. 135261-18-6;
Carbamodithioic acid, (1-methylethyl)-, monosodium salt, CAS Reg. No. 38457-16-8;
Carbamodithioic acid, (1-methylethyl)-, monopotassium salt,CAS Reg. No. 172539-30-9;
Carbamodithioic acid, dimethyl-, sodium salt, CAS Reg. No.128-04-1;
Carbamodithioic acid, dimethyl-, potassium salt, CAS Reg. No. 128-03-0;
Carbamodithioic acid, diethyl-, sodium salt, CAS Reg. No. 148-18-5;
Carbamodithioic acid, diethyl-, potassium salt, CAS Reg. No. 3699-30-7;
Carbamodithioic acid, dipropyl-, sodium salt, CAS Reg. No. 4143-50-4;
Carbamodithioic acid, dipropyl-, potassium salt, CAS Reg. No. 3699-31-8;
Carbamodithioic acid, bis(1-methylethyl)-, potassium salt, CAS Reg. No. 3803-9-3; and
Carbamodithioic acid, bis(1-methylethyl)-, sodium salt, CAS Reg. No. 4092-82-4.
The compounds of Formula I and Formula II are either available commercially or are capable of being synthesized according to techniques known to persons of ordinary skill in the art. Certain of the compounds of Formula I and Formula II are available from NALCO under the names Nalmet(copyright)8154 and Nalco(copyright)7614.
Preferable polymeric dithiocarbamates are of Formula III(polymeric saturated amine modified with carbon disulfide such that about 30 mole % dithiocarbamate salt groups are formed) and Formula IV(polymeric saturated amine modified with carbon disulfide such that about 50 mole % dithiocarbamate salt groups are formed). 
In Formula III, t is an integer from 2 to 45. R11 is a positive metal cation selected from the group consisting of sodium and potassium.
In Formula IV, q is an integer from 3 to 55. R12 is a positive metal cation selected from the group consisting of sodium and potassium.
Preferred polymeric dithiocarbamates of Formula III and IV have a weight average molecular weight of between about 500 and about 100,000.
Most Preferred polymeric dithiocarbamates of Formula III and Formula IV are available from NALCO under the names Nalmet(copyright)8702 and Nalmet(copyright)1689 respectively.
The amount of organic sulfide chelant added is from about 0.002% to about 0.02% by weight based on the total dry weight of the pulp, preferably from about 0.004% to about 0.008% by weight based on the total dry weight of the pulp, most preferably about 0.005% based on the total dry weight of the pulp.
It has been found that when an organic sulfide chelant is added to either a Chemical pulp or a Mechanical pulp before or during bleaching that brightness after bleaching is increased. Brightness is a term used to describe the whiteness of pulp on a scale: from 0%, meaning absolute black to 100%, relative to MgO standard, which has an absolute brightness of ca. 96%; by the reflectance of blue light (457 mm) from the paper produced from the pulp.
The unexpected finding of the instant claimed invention is that using less, (xe2x89xa6 about 0.02 weight %) of the organic sulfide chelant is preferable to using more. This is in contrast to the teachings of the Japanese Patent Application No. 4-114853 which recommends using a minimum of 0.04 weight % of the polymer.
It has also been found that when the organic sulfide chelant is used, that it is also possible to further enhance the brightness by the use of an optional additional chelant, an optional surfactant, and optionally polyacrylic acid.
The additional chelants are selected from the group consisting of organic phosphonates.
These organic phosphonates are known to people of ordinary skill in the art of pulp and papermaking.
The surfactant is selected from the group consisting of alkanol alcoxy sulfates, preferably sodium laurylether (trisethyleneoxy) sulfate. This compound is available commercially. Polyacrylic acid is available from Nalco as Nalco(copyright) PR-4512.
The amount of additional chelant used is known to people of ordinary skill in the art of pulp making.
The amount of surfactant used is known to people of ordinary skill in the art of pulp making.
The amount of polyacrylic acid used is known to people of ordinary skill in the art of pulp making.
The additional chelant or surfactant or polyacrylic acid is added before or contemporaneously with the organic sulfide chelant agent.
Optionally, Chemical pulps are subjected to pressurized oxygen delignification before or during the overall bleaching process. Pressurized oxygen delignification takes place using a liquor containing sodium hydroxide with pressurized oxygen. In a preferred version of this embodiment, the liquor includes an additive such as an organic phosphonate and/or a surfactant.
Delignification is also known as xe2x80x9clignin removalxe2x80x9d and is characterized by the kappa number of the pulp. The kappa number of the pulp is the volume, usually reported in ml of 0.1N potassium permanganate solution consumed by one gram of moisture-free pulp under the conditions specified in TAPPI Method T236om-99. The kappa number is linearly proportional to the amount of residual lignin in the pulp. The Pulp viscosity is determined by the capillary viscometer method, TAPPI Method T23om-99.
It has been found that when an organic sulfide chelant is added to a Chemical pulp prior to pressurized oxygen delignification, that the delignification is enhanced. The same organic sulfide chelants useful in improving brightness are useful in enhancing oxygen delignification. It is also true that the same amount of organic sulfide chelant is used to enhance oxygen delignification as is used to enhance brightness.
The second aspect of the instant claimed invention is in a method for making Mechanical pulp comprising the steps of grinding or refining wood to create unbleached pulp and then bleaching the pulp; the improvement comprising treating the pulp with an organic sulfide chelating agent either before bleaching, or during bleaching.
The Mechanical pulp can be one of several types including Stone GroundWood and Pressurized GroundWood pulp, RMP, TMP and TRMP.
Organic sulfide chelants suitable for use in this second aspect of the instant claimed invention are the same as those listed for use in the first aspect of the instant claimed invention. The preferred organic sulfide chelants for use in Mechanical pulps are Nalmet(copyright) 8702 and Nalmet(copyright)8154.
The amount of organic sulfide chelant added is from about 0.002% to about 0.02% by weight based on the total dry weight of the pulp, preferably from about 0.004% to about 0.008% by weight based on the total dry weight of the pulp, most preferably about 0.005% based on the total dry weight of the pulp.
As was stated previously, it has been found that when an organic sulfide chelant is added to a Mechanical Pulp that brightness after bleaching is increased.
The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill how to make and use the invention. These examples are not intended to limit the invention or its protection in any way.